Transient response of networks employed in protective relaying

Series and parallel combinations of resistance and inductance are investigated theoretically under maximum-transient conditions, to find alternative representations of the replica impedance employed in distance-protection applications for which the line current is converted to an equivalent voltage. When the primary-system-impedance angle exceeds the replica-impedance angle, the parallel configuration is shown to develop a significantly reduced transient component of the output voltage, and a lower transient flux is required in the core of the energising current transformer. The parallel-tuned-filter circuit is employed in differential current-protection schemes to allow a reduction in the transient component of the relaying quantity derived via a current transformer. The circuit parameters are investigated with a view to minimising the transient-flux requirements of the current transformer and developing an output approaching proportionality with the a.c. component of the primary current. The treatment includes energisation at nonresonant frequencies.